The vastus medialis muscle is one of the four heads of the quadriceps muscle.

Vastus medialis is the medial head of the quadriceps muscle.
The lower part of the vastus medialis muscle has oblique fibres, and is called the vastus medialis obliquus or VMO muscle. It has particular importance in stabilising the patella.
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CLINICAL PUBLICATIONS
The effect of McConnell taping on vastus medialis and lateralis activity during squatting in adults with patellofemoral pain syndrome. Lee S-E and Cho S-H. J Exerc Rehabil. 2013 Apr; 9(2): 326–330.